A spokesman for the White House says the United States is not conducting or coordinating with Russia on military operations in Syria, ahead of a visit to Russia by U.S. Secretary of State John Kerry.

White House spokesman Josh Earnest told reporters Thursday that "there is some speculation that an agreement may be reached [during Kerry's visit], but it's not clear that that will happen."

Kerry arrived in Moscow on Thursday.

Earnest also said Russia needs to decide whether it will focus on fighting extremists in Syria or propping up the government of Syrian President Bashar al-Assad.

"Trying to accomplish both goals is a fundamental contradiction," Earnest said, "because to prop up the Assad regime worsens the chaos inside of Syria, and extremist organizations rely on that chaos to thrive."

FILE - Syrian President Bashar al-Assad

Earlier Thursday, Assad said Russia has never asked him to relinquish power and step down as president.

In an interview with NBC News, Assad said neither Russian Foreign Secretary Sergey Lavrov nor President Vladimir Putin have asked him to give up power, contrary to speculation that a back-channel agreement between Russia and the United States called for him to leave office.

"The Russian politics is not based on making deals," Assad said. "It is based on values."

The Syrian leader said Russia has never "said a single word" regarding him leaving office. "Only the Syrian people define who is going to be president," he told his interviewer. "When to come and when to go."

On his visit to Moscow this week, Kerry is expected to push for a revised cease-fire in Syria and the so-called "roadmap" to peace.

A U.S.-led coalition is carrying out airstrikes against extremists in Syria and Iraq, and Islamic State militants have lost significant amounts of territory in recent months.

In the interview, Assad dismissed the U.S. efforts as "not serious" and said "that military ineffectiveness is a reflection of their political will."

"The reality is telling us that since the beginning of the American airstrikes the terrorism has been expanding and pervading," Assad said. "It has only shrinked when the Russians intervened."